Somerville’s Union Square won’t be the same once the Green Line Extension opens in three years, and that’s the idea. The trains are supposed to bring sweeping changes to Union Square. But battle lines are forming over what kind of changes are coming, and the outcome will determine whether the Green Line serves as a tool for growth, or just for runaway gentrification.

If the maneuvering over the redevelopment of a shuttered Union Square social center is any indication, the battle between growth and gentrification is shaping up to be far more closely fought than it ought to be.

The old Boys and Girls Club building on Washington Street sits on the edge of Union Square, close to a rusting highway overpass. The new Green Line route will bring this part of Somerville within a few minutes from downtown Boston, and turn the neighborhood into one of the region’s hottest development draws. The nonprofit Somerville Community Corporation has been working for more than two years to turn the old Boys and Girls Club building into an early bulwark against the waves of market pressure that the Green Line will bring.

The group partnered with a private Boston developer, and together they proposed replacing the Boys and Girls Club and a funeral parlor with a two-building, 84-unit apartment development. The project would contain a high number of affordable housing units, and would be among the first to take advantage of new Green Line zoning.

Neighbors balked at the proposal. They complained about the same things that virtually anyone who lives next to new construction around Boston complains about — building height, traffic, noise, and parking. The developers cut the size of its proposed development down to 74 units, which was good enough to win over Somerville’s Planning Board. The development’s neighbors weren’t so easily appeased. A few sued.

Trips to the Land Court are brutal diversions for developers. The community developer couldn’t wait its lawsuit out, so it designed a smaller project in the hopes of winning over critics. Somerville’s Planning Board will soon vote on a smaller, 65-unit housing development for Washington Street. The proposed building won’t lose any of the 35 affordable units it’s permitted for now, but chopping a floor off its permitted height will force the community developer to shed some of the family-sized apartments it had planned to build.

Somerville has big plans for the real estate surrounding the future Green Line tracks. It upzoned Union Square, and is now weighing bids from developers looking to build millions of square feet of offices, housing, and retail in the square. The city pushed hard to level the aging McGrath-O’Brien Highway overpass at Washington Street, and beyond the overpass, massive redevelopments of the city’s Brickbottom and Inner Belt sections are taking shape.

But Somerville’s Green Line bet only works if developers can take advantage of the opportunities the streetcars present. The Somerville Community Corporation’s Washington Street development will be one of the first built under the city’s new Green Line zoning. Its experience shows the same old housing fights persisting, even after years of community dialogue around Union Square’s future. The 65-unit compromise the neighborhood developer is now seeking will be well below zoning allowances.

Somerville can’t afford to make a habit of permitting transit-oriented developments that fall short of zoning. A new report by the Metropolitan Area Planning Council warns that the Green Line extension brings the risks of significant rent spikes and waves of condominium conversions. To blunt those waves and maintain current levels of affordability, the report says, Somerville needs developers to produce as many as 9,000 new housing units by the year 2030.

The city watched skyrocketing prices and wholesale resident turnover follow the Red Line to Davis Square in the 1980s. Neighborhoods like Union Square and the Inner Belt enjoy one advantage over Davis Square: They have enough land to produce the new housing that’s needed to blunt the market pressures the Green Line will bring.

Still, it’s one thing to have the elbow room to get aggressive on new housing construction, and another thing to actually do it. Somerville’s residents and planners have already put in years laying the zoning groundwork that will allow the city to use the Green Line to make big, exciting things happen, while cushioning gentrification pressures. Now it’s up to neighborhood residents to let developers follow through.

Paul McMorrow is an associate editor at Commonwealth Magazine. His column appears regularly in the Globe.

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